Aluminium salts are well known in the art as providing a safe excipient with adjuvant activity. The mechanism of action of these adjuvants are thought to include the formation of an antigen depot such that antigen may stay at the site of injection for up to 3 weeks after administration, and also the formation of antigen/metallic salt complexes which are more easily taken up by antigen presenting cells. In addition to aluminium, other metallic salts have been used to adsorb antigens, including salts of zinc, calcium, cerium, chromium, iron, and berilium. The hydroxide and phosphate salts of aluminium are the most common.
Vaccine formulations containing aluminium salts, antigen, and additional immunostimulant are known in the art. Such formulations induced greater immune responses in comparison with those stimulated by aluminium salts and antigen alone. Formulation of these vaccine preparations have previously involved a specific manufacturing procedure, since it was believed that in order for optimal immune responses to occur, the antigen must be adsorbed onto the same aluminium salt particle as the immunostimulant. In this way when antigen is taken up by an antigen presenting cell, the co-adsorbed immunostimulant exerts its stimulatory activity directly onto that same antigen presenting cell.
Aluminium based vaccine formulations wherein the antigen and the immunostimulant 3-de-O-acylated monophosphoryl lipid A (3D-MPL), are adsorbed onto the same particle are described in EP 0 576 478 B1, EP 0 689 454 B1, and EP 0 633 784 B1. In these cases then antigen is first adsorbed onto the aluminium salt followed by the adsorption of the immunostimulant 3D-MPL onto the same aluminium salt particles. Such processes first involve the suspension of 3D-MPL by sonication in a water bath until the particles reach a size of between 80 and 500 nm. The antigen is typically adsorbed onto aluminium salt for one hour at room temperature under agitation. The 3D-MPL suspension is then added to the adsorbed antigen and the formulation is incubated at room temperature for 1 hour, and then kept at 4° C. until use.
The formulation processes of the prior art provide for potent vaccines from an immunological point of view, however, they do contain several commercial disadvantages. In order for a vaccine to be suitable for human administration, the process must be uniform and be subject to Good Manufacturing Practice (GMP) control, and Quality Control (QC). In some cases the processes of the prior art provide a vaccine wherein all of the antigen, or antigens, are adsorbed onto the same particle of metallic salt. The process is then complicated by the requirement for the 3D-MPL to be adsorbed onto the same metallic particle. This may be particularly problematical in the case of combination vaccines containing multiple antigens (whose adsorption may be dependent on the affinity of each antigen to the particular metallic salt at a given pH). The processes of the prior art may have problems, depending on which antigens are present, in reproducibility and vaccine QC. Furthermore, if anything undesired occurs with the QC of one particular antigen, or an occurrence which may result in the contamination of the vaccine, this may result in the waste all of the individual components, and not just the particular antigen in which the problem occurred. Moreover, in some circumstances combination vaccines may require the sequential addition of the antigens, such a process being hugely time consuming and expensive. The processes of the prior art may, therefore, be complex, difficult to control, and expensive.